It is commonly known that drilling mud is a heterogeneous liquid system wherein colloid-size particles of the solid phase are always present. The presence of these particles is, from the well-drilling quality point of view, an essential prerequisite of adequate rheological properties of the drilling mud. The latter is expected to retain these qualities, essential as they are for optimization of the well-drilling conditions. However, stable retaining of the essential properties of the drilling mud throughout a drilling operation has been presenting a complicated problem.
The majority of drilling operations are carried out in clayey rock. The clayey rock being drilled is partly finely disintegrated, and the rock cuttings in the form of colloid particles get into the drilling mud.
Insufficiently effective purification of the drilling mud from rock cuttings upon several cycles of mud-pumping in the course of a well-drilling operation has been found to significantly alter the composition of the solid phase of the drilling mud, which necessitates using various methods of enhancing the drilling mud quality. Therefore, the quality purification of the drilling mud from rock cuttings is of paramount importance for the well-drilling operation.
Poor quality of the purification of the drilling mud has been a major cause of various emergencies and complications involving losses of the drilling mud, sticking of drill-pipes and casings, caving-in of rock from the borehole walls.
The technical and economic ratings of a drilling operation are greatly influenced by the quality of the drilling mud used, as well as by the degree of its purification from rock cuttings.
Quality purification of the drilling mud helps getting a well down faster, owing to the reduced content of the solid phase in the liquid one, and enhances the working environment of the bits and down-hole tools. Apart from stepping up the mechanical rate of drilling, quality purification of the drilling mud helps reducing the consumption of the materials spent on maintaining the required properties of the drilling mud, prolonging the life of the mud, avoiding emergencies and complications of the drilling operation.
In short, quality purification of the drilling mud from rock cuttings is an essential process of the well-drilling operation, significantly influencing technical and economic ratings of the operation.
All the hitherto known techniques of purifying drilling muds enable to remove from the circulating mud a certain proportion of solid particles with a certain degree of efficiency and quality.
Thus, the minimum size of particles that can be separated from a drilling mud on vibrating sieves is defined by the mesh size of the sieve. With the finer mesh used to enhance the quality of the purification, the pass-through capability of the sieve becomes seriously affected, and the loss of the drilling mud with the sludge is stepped up.
When purified in hydrocyclones, the drilling mud is diluted with water, and particles of a relatively high density are predominantly removed therefrom. The finer or less dense particles which become resident in the drilling mud as rock cuttings become dispersed therein would not be removed in hydrocyclones and numerous other purification devices currently in use.
There is known a method of regeneration of a stable clayey suspension of a drilling mud, according to which the drilling mud coming from the well and containing rock cuttings is prediluted with water and has greater particles removed therefrom. The thus diluted and precleaned drilling mud contains fine uncharged particles of cut rock and negatively charged colloidsize particles of clay. Then the negatively charged particles of clay are separated from the mud by being deposited on a rotating anode and subsequently removed with a scraper tool, the settling negatively charged particles of clay entraining therewith some of the uncharged particles which are likewise deposited on the rotating anode (Cf. the CZ Patent No. 109,992; Cl. 5, a, 31/20, dated Feb. 2, 1964).
The abovedescribed method is used to purify but a portion of the total flow of the drilling mud, while the far greater remaining portion of the mud is recirculated into the well in an uncleaned state. Furthermore, the stagewise purification, first, from the coarser particles, and, then, from the finer one is complicated and, hence, costly.